1) Field of the Invention
The invention relates to low-emission adhesives based on an aqueous, protective-colloid-free polymer dispersion or to water-redispersible dispersion powders, obtainable therefrom, and also to vinyl acetate-ethylene copolymers.
2) Background Art
It is known to use dispersions based on acrylate polymers as pressure-sensitive adhesives and flooring adhesives. These dispersions are suitable for applications as pressure-sensitive and flooring adhesives since owing to the low glass transition temperature of the acrylate polymers, usually based on butyl acrylate and/or 2-ethylhexyl acrylate, the required surface tack is achieved.
EP-A 17896 (U.S. Pat. No. 4,322,516) discloses vinyl acetate-ethylene copolymers which contain a high proportion of acrylate and which exhibit good surface tack even without the addition of tackifier resins. EP-A 185356 (U.S. Pat. No. 4,831,077) and EP-A 216210 (U.S. Pat. No. 4,997,879) relate to vinyl acetate-ethylene copolymers which have an acrylate fraction and which owing to the copolymerization of hydroxy-functional comonomers still exhibit excellent adhesion even at elevated temperatures.
On an alkaline substrate at a pH  greater than 9.0, however, as a result of hydrolysis of the ester group, acrylate-containing dispersions give off the corresponding alcohols. For instance, butyl acrylate gives off butanol and 2-ethylhexyl acrylate gives off 2-ethylhexanol, which may be emitted to the ambient air. Since this hydrolysis cannot be stopped, these alcohols will be emitted to the environment from the applied dispersion over a period of weeks, months and years.
The object of the invention is therefore to develop an adhesive dispersion with which this emissions problem does not arise but which in terms of surface tack, adhesion, and tackiness, is characterized by properties comparable with that of acrylate-containing dispersions.
This object has been achieved by replacing the acrylate fraction in the copolymer with branched vinyl ester units.
From EP-A 699692 (U.S. Pat. No. 5,665,816) it was known, for the purpose of improving the adhesion on untreated polyolefin substrates, to use adhesive dispersions containing cellulose ethers grafted onto vinyl acetate-ethylene copolymers which also have acrylate fractions and fractions of branched vinyl esters. In EP-A 841351, it is recommended that the acrylates be copolymerized with vinyl esters of neo acids in order to improve the adhesion of polyacrylate-based pressure-sensitive adhesives on nonpolar surfaces. U.S. Pat. No. 5,371,137 describes the use of vinyl acetate-ethylene copolymers modified with long-chain vinyl esters for improving the adhesion on nonpolar surfaces.
The invention provides low-emission adhesives based on an aqueous, protective-colloid-free polymer dispersion or water-redispersible dispersion powders, obtainable therefrom, of vinyl acetate-ethylene copolymers, obtainable by free-radically initiated emulsion polymerization, in an aqueous medium and in the presence of one or more emulsifiers, of a comonomer mixture comprising
a) from 5 to 50% by weight of ethylene,
b) from 20 to 80% by weight of one or more vinyl esters from the group of vinyl esters of unbranched or branched carboxylic acids having 1 to 9 carbon atoms whose homopolymers have a glass transition temperature Tg  greater than 0xc2x0 C.,
c) from 5 to 70% by weight of one or more vinyl esters from the group of vinyl esters of branched carboxylic acids having 8 to 13 carbon atoms whose homopolymers have a glass transition temperature Tg  less than 0xc2x0 C.,
d) from 0.5 to 10% by weight of one or more ethylenically unsaturated monocarboxylic or dicarboxylic acids having 3 or 4 carbon atoms,
e) from 0 to 10% by weight of one or more ethylenically unsaturated, hydroxyalkyl-functional comonomers,
f) from 0 to 10% by weight of further, mono- or polyethylenically unsaturated comonomers,
the % by weight being based in each case on the overall weight of the comonomers and adding up to 100% by weight, and the dispersion obtained therewith being dried if desired.
It is preferred to copolymerize from 10 to 40% by weight of ethylene.
Suitable vinyl esters b) from the group of vinyl esters of unbranched or branched carboxylic acids having 1 to 9 carbon atoms whose homopolymers have a glass transition temperature Tg  greater than 0xc2x0 C. are vinyl acetate, vinyl propionate, vinyl butyrate, 1-methylvinyl acetate, vinyl pivalate, and vinyl esters of xcex1-branched monocarboxylic acids having 9 carbon atoms (VeoVa9(copyright), a trade name of the Shell company). Preference is given to vinyl acetate, vinyl propionate, and VeoVa9(copyright). Particular preference is given to vinyl acetate. Most preferably, from 45 to 70% by weight of vinyl esters b) are copolymerized.
Suitable vinyl esters c) from the group of vinyl esters of branched carboxylic acids having 8 to 12 carbon atoms whose homopolymers have a glass transition temperature Tg  greater than 0xc2x0 C. are vinyl 2-ethylhexanoate, vinyl esters of xcex1-branched monocarboxylic acids having 10 or 11 carbon atoms (VeoVa10(copyright), VeoVa11(copyright) trade names of Shell), and vinyl esters of branched monocarboxylic acids having 10 to 13 carbon atoms (Exxar Neo12). Preference is given to the vinyl esters of xcex1-branched monocarboxylic acids having 10 or 11 carbon atoms (VeoVa10(copyright), VeoVa11(copyright)). Most preferably, from 10 to 45% by weight of vinyl esters c) are copolymerized.
Suitable ethylenically unsaturated monocarboxylic and dicarboxylic acids d) are acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid, preference being given to acrylic acid, methacrylic acid, and itaconic acid. Acrylic acid and methacrylic acid are particularly preferred. Most preferably, from 2 to 6% by weight of comonomers d) are copolymerized.
Suitable ethylenically unsaturated, hydroxyalkyl-functional comonomers e) are methacrylic and acrylic hydroxyalkyl esters having a C1 to C5 alkyl radical such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate; preferably, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate. Most preferably, from 0 to 5% by weight of comonomers e) are copolymerized.
Examples of mono- or polyethylenically unsaturated comonomers f) are functional comonomers such as ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile; monoesters and diesters of fumaric acid and maleic acid such as the diethyl and diisopropyl esters and also maleic anhydride, ethylenically unsaturated sulfonic acids and their salts, preferably vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid. Further examples are pre-crosslinking comonomers such as polyethylenically unsaturated comonomers, examples being divinyl adipate, diallyl maleate, allyl methacrylate and triallyl cyanurate or postcrosslinking comonomers, examples being N-methylolacrylamide (NMA), N-methylolmethacrylamide, alkyl esters such as the isobutyoxy ether or esters of N-methylolacrylamide. Also suitable are epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate. Further examples are silicon-functional comonomers, such as acryloxypropyltri(alkoxy)- and methacryloxypropyltri(alkoxy)-silanes, vinyltrialkoxysilanes, and vinylmethyldialkoxysilanes.
The preferred auxiliary monomers f) are ethylenically unsaturated carboxamides such as acrylamide and methacrylamide, ethylenically unsaturated sulfonic acids and their salts such as vinylsulfonic acid and vinylpyrrolidone. Acrylamide is particularly preferred. Most preferably, from 0 to 2% by weight of comonomers f) are copolymerized.
In the copolymers, the amounts in percent by weight add up in each case to 100% by weight. In general, the monomers and/or the weight fractions of the comonomers, are selected so as to give a glass transition temperature Tg of less than 0xc2x0 C., preferably, from xe2x88x9260xc2x0 C. to xe2x88x9210xc2x0 C.
The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately in advance using the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956), the following is true: 1Tg=X1Tg1+x2/Tg2+ . . . +xn/Tgn, where xn is the mass fraction (% by weight/100) of the monomer n, and Tgn is the glass transition temperature, in kelvins, of the homopolymers of the monomer n. Tg values for homopolymers are listed in Polymer Handbook, 2nd edition, J. Wiley and Sons, New York (1975).
Particularly preferred copolymers consist of the following comonomer units:
from 10 to 40% by weight of ethylene, b) from 35 to 70% by weight of vinyl acetate, c) from 10 to 45% by weight of VeoVa10(copyright) and/or VeoVa11(copyright), d) from 2 to 6% by weight of acrylic acid and/or methacrylic acid, e) from 0 to 5% by weight of hydroxyethyl acrylate, and f) from 0 to 2% by weight of acrylamide.
Preparation by the emulsion polymerization process takes place at from 30xc2x0 C. to 90xc2x0 C., preferably from 45xc2x0 C. to 70xc2x0 C., and under a pressure of from 30 to 100 bar abs., preferably from 40 to 80 bar abs. The polymerization can be conducted by the batch technique where all components are included in the initial charge to the reactor and by the metering technique where individual components, or a number of components are fed in during the polymerization. The feeds can be made separately (spatially and temporally), or the components to be metered in can be metered in in whole or in part in pre-emulsified form. Preference is given to the batch technique wherein the comonomer components a), b) and c) are included in the initial charge and initiators, emulsifiers and any other comonomers d), e) and f) are metered in.
The polymerization is initiated using the initiators or redox initiator combinations generally for emulsion polymerization; examples are hydroperoxides such as tert-butyl hydroperoxide, tert-butyl peroxopivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide, azo compounds such as azobisisobutyronitrile, inorganic initiators such as the sodium, potassium and ammonium salts of peroxodisulfuric acid. The initiators are generally used in an amount of from 0.05 to 3% by weight based on the overall weight of the monomers. The redox initiators used are combinations of said initiators with reducing agents. Suitable reducing agents are the sulfites and bisulfites of the alkali metals and of ammonium, an example being sodium sulfite, the derivatives of sulfoxylic acid such as zinc or alkali metal formaldehyde sulfoxylates such as sodium hydroxymethanesulfinate (Rongalit(copyright)), and ascorbic acid or isoascorbic acid. The amount of reducing agent is preferably from 0.01 to 5.0% by weight based on the overall weight of the monomers.
Preference is given to redox catalyst combinations comprising sodium, potassium or ammonium salts of peroxodisulfuric acid as the oxidizing component and ascorbic acid or iso-ascorbic acid as the reducing agent.
The polymerization batch is stabilized using emulsifiers with the exclusion of protective colloids. The overall amount of emulsifier is preferably from 0.1 to 5% by weight, in particular, from 0.5 to 3% by weight, based on the overall weight of the comonomers. Suitable emulsifiers are anionic or non-ionic emulsifiers or mixtures thereof, examples being the following:
1) Alkyl sulfates, especially those having a chain length of 8 to 18 carbon atoms; alkyl and alkylaryl ether sulfates having 8 to 18 carbon atoms in the hydrophobic radical and from 1 to 50 ethylene oxide units.
2) Sulfonates, especially alkyl sulfonates having 8 to 18 carbon atoms; alkylaryl sulfonates having 8 to 18 carbon atoms, esters and monoesters of sulfosuccinic acid with monohydric alcohols or alkylphenols having 4 to 15 carbon atoms in the alkyl radical; if desired, these alcohols or alkylphenols may also have been ethoxylated with from 1 to 40 ethylene oxide units.
3) Phosphoric acid partial esters and their alkali metal salts and ammonium salts, especially alkyl and alkylaryl phosphates having 8 to 20 carbon atoms in the organic radical, alkyl and alkylaryl ether phosphates having 8 to 20 carbon atoms in the alkyl or alkylaryl radical, respectively, and from 1 to 50 EO units.
4) Alkyl polyglycol ethers preferably having from 8 to 40 EO units and with alkyl radicals having 8 to 20 carbon atoms.
5) Alkylaryl polyglycol ethers preferably having from 8 to 40 EO units and having 8 to 20 carbon atoms in the alkyl and aryl radicals.
6) Ethylene oxide/propylene oxide (EO/PO) block copolymers preferably having from 8 to 40 EO and/or PO units.
Preference is given to alkyl and alkylaryl ether sulfates having 8 to 18 carbon atoms in the hydrophobic radical and from 1 to 50 ethylene oxide units such as sulfated nonylphenol ethoxylates and sulfated isotridecyl ethoxylates.
For adjusting the molecular weight, it is possible during the polymerization to add the regulators commonly used, examples being n-dodecyl mercaptan, t-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol, and acetaldehyde, preferably in amounts of from 0.1 to 3.0% by weight based on the overall weight of the monomers.
The aqueous dispersions obtainable by the process of the invention have a solids content of from 30 to 75% by weight, preferably from 40 to 65% by weight. To prepare the water-redispersible dispersion powders, the aqueous dispersions are dried in a known manner, by means for example of fluidized-bed drying, freeze drying, and, preferably, spray drying.
Adhesives are suitable in particular for applications as pressure-sensitive adhesives and as flooring, wall and ceiling adhesives. The preferred field of application is that as a flooring adhesive for bonding elastic and textile coverings. Particular preference is given to their application as low-emission flooring adhesives. Appropriate formulations for flooring adhesives generally include from 15 to 50% by weight of dispersion, from 5 to 35% by weight of tackifier resin, from 15 to 50% by weight of filler, the proportions in % by weight adding up in each case to 100% by weight. For wall and ceiling adhesives, this formulation may also be used in combination with natural thickeners such as cellulose and starch or with synthetic thickeners. In these two fields of application, the aforesaid formulation is often also used without the addition of tackifier resins. Typical formulations for pressure-sensitive adhesives are in many cases based only on the adhesive dispersion. If desired, tackifier resins or processing aids may be added.
What is advantageous about the claimed systems is that no alcohols such as butanol or 2-ethylhexanol can be released from the dispersion on alkaline substrates by hydrolysis and at the same time, the performance properties, in particular high surface tack, high adhesion (peel strength) and high cohesion (resistance to shear) of the acrylate-containing dispersions are attained. In contrast, Vac/E systems of comparable Tg exhibit low cohesion, adhesion and tack and poor spreadability in flooring adhesive formulations.
The examples which follow serve to illustrate the invention: